Costs for services grow, and how you pay? Nobody knows ...

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Are you looking forward to using this year's tax rebate for travel? Try not to spend it all on a plane ticket and a hotel. You'll need some of that cash just to get through the airport, especially if you plan to check luggage at the curb.

Are you looking forward to using this year's tax rebate for travel? Try not to spend it all on a plane ticket and a hotel. You'll need some of that cash just to get through the airport, especially if you plan to check luggage at the curb.

And if you're checking more than one bag at the curb, be sure to save some cash for some aspirin as well. Many airlines will roll out new excess baggage fees on May 5th. But two weeks out, it's still nearly impossible to determine how travelers will be able to pay those fees at the curb.

The curbside hustle
For many travelers, complimentary curbside baggage checks have been a much-appreciated convenience. Increasingly, though, airlines are charging what Delta has dubbed a "Curb Administrative Fee" for this service.

American Airlines, United Airlines, Northwest Airlines and US Airways, for example, have already instituted a $2 per bag fee (tip not included) for curbside bag check-in service. On April 1 — of all days — Delta upped the ante with its announcement of a $3-per-bag fee for curbside check-in at 11 “test” airports.

This past Tuesday (April 15 — of all days), Delta's $3 per bag fee spread nationwide, to the approximately 100 domestic airports where the airline offers curbside service.

It's a fair bet that other airlines will soon follow suit.

You can avoid this curbside check-in fee by going inside the terminal, of course, and standing in line with everyone else to check your bags at a ticket counter. If you do choose curbside check-in though, be sure to stop at the bank before you head to the airport: most services are cash-only operations. And even though it may not be spelled out at the counter, those airline-imposed check-in fees do not include any tip you may want to go directly to the skycap.

Coming soon: more curbside confusion
So far these curbside check-in fees have been very controversial for travelers and for the many skycaps who count on tips for a large part of their income. In fact, a federal jury recently ordered American Airlines to pay more than $325,000 to nine skycaps at Boston's Logan International Airport. They claimed that their tips were seriously eroded by the airline's curbside check-in fees. Now the lawyer for the Boston skycaps is considering filing a class-action suit on behalf of skycaps nationwide.

If you think the curbside check-in procedures are costly and confusing now, just wait until May 5th. That's when many airlines will begin charging an extra $25 for checking a second piece of luggage. The airlines are clear on the fees, but they're not quite clear yet on how the fees will be collected at the curb.

A well-mannered traveler's dilemma
I spent much of Sunday evening and Monday scouring airline Web sites and calling reservation agents (again and again) for an answer to this simple question:

“I'm traveling in mid-May with two suitcases. What will the fee be for checking those two bags at the curb? What type of payment will be accepted?”

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Well, apparently not. The information I was given was by turns, conflicting, confusing — or just plain wrong.

For example, one United Airlines reservation agent insisted repeatedly that “there are no charges to check bags at the curb.” How could he be so sure when the airline's Web site says “a small fee applies” for curbside check-in? “My supervisor confirms it.” When pressed to check again, he forwarded me to a Web support desk. There I was told there certainly was a fee, but that it was unclear if the fee was still $2. I was assured however, that if I wanted to check a second bag at the curb after May 5th, I could pay for that with a credit card, cash or personal check. An hour later, a different reservation agent told me that only credit cards or cash would be accepted.

At Northwest Airlines, the first reservation agent I spoke with told me she was “pretty sure” I'd need cash to pay for checking a second bag at the curb. When I questioned that “pretty sure,” and asked her to check with a supervisor, she came back on the line after about 10 minutes to tell me that only credit cards and checks would be accepted. I called back a few hours later and got a different answer and a lecture: According to this agent, Northwest Airlines sets the fees, but they do not actually employ the skycaps. “The airport does that so you'll need to call Travelers Aid to find out the name of the company if you want to know for sure how they handle the fees at each airport you go to.”

Can that be true? I'm still waiting on a call-back on that one.

Frustrated with the information I was getting from reservation agents at Delta Airlines, I turned to Susan Elliott, a Delta spokesperson. She assured me that, just as it says on the airline's Web site, as of April 15th, Delta now levies a $3 cash-only per-bag fee for curbside check-in. After May 5th, when the airline begins charging $25 for a second checked bag, Elliot says travelers will be able to pay for that at the curb with a credit card only. “We're in the process of communicating this new information to our customer service agents,” she told me, “so they'll be prepared to address any questions or concerns from passengers.” And while some agents may not be up to speed quite yet, Elliott is confident they'll all know the rules by May 5th.

For now, well-mannered travelers checking bags curbside at any airport and on any airline should arm themselves with credit cards, cash and plenty of patience. Especially after May 5th.

Here are a few other tips:

Keep checking your airline Web site. As questions come in and the May 5th date gets closer, information is sure to get updated.

Once you find the applicable baggage policy on your airline Web site, print it out and keep a copy with your travel papers. It may come in handy at the curb.

If you talk to a reservations agent, take notes about what you're told. If it sounds like the agent is making something up or is unsure of what they're telling you, ask them to please check with a supervisor. Be prepared to wait. And don't hesitate to call back again to reconfirm what you're told.

Pack light. Remember, you won't even need to worry about extra baggage fees or curbside check-in costs if you travel with just a carry-on suitcase. At least for now. There's no telling what new fees we'll wake up to tomorrow.

Harriet Baskas writes msnbc.com's popular weekly column, The Well-Mannered Traveler. She is the author of the “Stuck at the Airport” blog, a contributor to National Public Radio and a columnist for USATODAY.com.